It has been found that thermal comfort for an occupant in a vehicle is improved significantly when the air temperature is measured at the occupant""s breath-level. The present invention is directed to a control system that works in conjunction with an automatic climate control system to measure air temperature at or adjacent to a vehicle occupant""s breath-level and outputs a signal to the climate control system in order to improve the thermal comfort for the occupant. The present invention uses ultrasonic transducers to generate and receive acoustic waves that are propagated along a pre-selected path that passes adjacent to the vehicle occupant""s breath-level. The transmission time of the acoustic waves is used to determine an average air temperature. The present invention correlates well with the temperature measured by a thermocouple in air at breath-level. This invention may also be used to measure air flows within the vehicle cabin.
Automatic control of air temperature in a vehicle is made difficult both by the unpredictable nature of the cabin thermal environment and because people have different perceptions of thermal comfort. The factors that affect thermal comfort can be generally attributed to those environmental factors that affect body heat loss. In a vehicle interior, thermal comfort is mainly affected by the distribution of air temperature and air velocity around the occupant. Also important are the occupant""s clothing, the amount of radiant heating and the humidity in the passenger cabin. A vehicle as used here refers to an automotive vehicle such as a car, bus, truck, van, sport utility vehicle, recreation vehicle, etc, or to an aircraft.
Several systems are presently used in vehicles for automatic climate control (ACC). Most current vehicles utilize an in-car sensor, located in the instrument panel near the base of the steering wheel, which draws air through a port from the vehicle""s passenger cabin over a thermistor that measures the temperature of the air passing through the port. The sensor outputs the temperature to the vehicle""s heating, ventilation and air conditioning (HVAC) system to control heating and cooling. Some vehicles also use multiple thermistors located at various points in the vehicle interior, and then average the temperature throughout the cabin.
In use, the ACC system has transient modes, such as warm-up and cool-down. During steady state operation, it tries to maintain the interior temperature at a desired set point. Present in-car temperature sensors, however, are prone to response-time lag and drift. This leads to control problems during some transient modes and even under steady state conditions.
More recently, some vehicles have begun using an infrared detector that measures the average surface temperature of persons or objects in a selected field of view based on a thermistor that measures air temperature and a thermopile detector that measures radiant flux from the field of view, such as, for example, disclosed in U.S. Pat. No. 5,518,176. Such systems, however, tend to be somewhat unreliable in accurately measuring air temperature because the temperature of a solid surface in a vehicle""s interior changes slowly in response to changes in air temperature. Therefore, during a fast transient, the output from a sensor that measures the temperature of solids, such as an infrared detector, does not correlate well with the actual air temperature in the vehicle.
Models are known in the art for determining and controlling the temperature distribution inside a vehicle cabin using various inputs to the ACC controller. In addition to the in-car thermistor or infrared-based sensor, other inputs to current ACC systems include sensors of outside air temperature and sunlight exposure to the cabin, known as solar heat load. The desired temperature setting selected by each occupant is yet another input factor, or a default setting if none is selected by some occupants. Additional inputs include interior vehicle air flows, air vent discharge temperature, the thermal history of the vehicle, and the presence of occupants. The present invention may be used to replace the current in-car thermistor-based or infrared-based sensor and obtain a much improved measurement of bulk air temperature near or adjacent to the breath-level of one or more vehicle occupants. By accurately and rapidly measuring the average air temperature and average air velocity along a pre-selected path, the models known in the art can be used to find the temperature distribution in a vehicle cabin more accurately and more quickly than is currently done with the in-car sensor. It has been surprisingly found, however, that thermal comfort of a vehicle""s occupant correlates very well with the temperature measured by a thermocouple, hanging in the air, positioned at breath-level in front of that occupant. It has further been found that the breath-level measurement is substantially better than the measurement of air temperature using in-car sensors or infrared temperature detectors known in the art.
The present invention is directed to an acoustic HVAC control system that determines the average air temperature along a pre-selected path to provide more accurate and timely climate control of a vehicle interior. It has been found that existing temperature measurement provides a poor correlation to thermal comfort of an occupant and that the correlation is much better when the temperature is measured at breath-level in front of the occupant""s (such as the driver""s) face. However, until the present invention, temperature measurement at such a location has not been feasible.
The present invention finds good agreement between breath-level temperature and the average air temperature along a pre-selected path even when the path is adjacent to the occupant and not directly in front of the occupant""s face. For example, good agreement is found when the path is along the occupant side of the roof console that is mounted on the headliner near the occupant. Good agreement is also found when the path is along the top of the instrument panel or between the xe2x80x9cAxe2x80x9d pillar near a front seat occupant and the rear view mirror. Further, the path could be along the side of the occupant""s seat or headrest, a rear view mirror, on the headliner above the occupant or from one part of the instrument panel to another. For the particular case of the vehicle""s driver, the path is preferably from one part of the steering wheel or steering column to the another or to the instrument panel. For an occupant of a rear seat, the acoustic path is preferably from one part of the seat-back in front of the occupant to another (e.g., top-to-bottom or side-to-side). For an occupant of an aircraft seated in a seat on one side of the aircraft (that is, a window seat), the acoustic path is preferably parallel to and near that side of the aircraft. Thus, the exact location of the path adjacent to the occupant or to the occupant""s breath-level is less important in that the path averages the air temperature near or adjacent to the occupant""s breath-level, thus measuring the local average air temperature and not just an average temperature of the air in the entire vehicle. This local measurement can then be repeated if desired for other locations in the vehicle to facilitate zone control by the ACC system to improve thermal comfort for occupants in those locations.
It is however important that most of the path of the acoustic wave be spatially separated from the solid surfaces in the vehicle. This is because of the thermal boundary layer in the air near such surfaces. At such surfaces, the air velocity is zero and the air temperature is the same as the temperature of the solid. Outside of the boundary layer, which can for example be as much as 1 centimeter thick under some circumstances in the vehicle, turbulent flow tends to equalize the bulk air temperature. Thus, it is desirable to have the majority of the acoustic path be outside the boundary layer where it measures the bulk air temperature. Obviously, the part of the boundary layers which are associated with the solid surfaces of an ultrasonic transducer and an acoustic reflector are inevitably part of the acoustic path. In the present invention, an acoustic path length (between the transducer and the reflector) is contemplated to be in the range of about 10 cm to about 200 cm, with an acoustic path length of about 50 cm being preferred.
This understanding of the thermal boundary layer also helps to appreciate a problem with the usual thermistor-based in-car sensor. Being mounted inside a protective enclosure which is typically on the vehicle instrument panel, the in-car sensor is inside the boundary layer and thus does not do a good job of measuring the air temperature in the vehicle or near the vehicle occupant. Similarly, an infrared-based system measures the temperature of a solid surface, which is inside the boundary layer. These problems are especially important during thermal transients, such as during the period of warming up a cold-soaked vehicle or cooling down a hot-soaked vehicle. In these cases, the fast measurement time of the present invention, which can be much faster than one second, is an additional advantage.
As mentioned above, this acoustic measurement technique can be repeated for other vehicle occupants in different positions or zones. Alternatively, a measurement of average air temperature along a pre-selected path adjacent to the breath-level of one vehicle occupant using this acoustic technique can be used to infer the temperature near other vehicle occupants using other inputs to the automatic climate control system and models known in the art. These other inputs may include solar heat load (including the direction of the sun relative to the vehicle), outside air temperature, interior vehicle air flows, air vent discharge temperature, a setpoint temperature which was pre-selected by another occupant or by a default value, the thermal history of the vehicle, and the presence of another occupant. An average air velocity along the pre-selected path can also be found by its effect upon the difference in interval time for acoustic pulses traveling in opposite directions along the pre-selected path. This furnishes additional useful input to the ACC to aid in modeling the thermal environment of the occupants.
The present invention is based on the principle that the speed of sound depends on air temperature and that average air temperature along a path is given by the time that an acoustic pulse takes to pass between two fixed points. In a vehicle cabin, the relationship is essentially that sound velocity is proportional to the square root of the air temperature (in degrees Kelvin).
Sound propagates at constant velocity c away from a source. The dependence of the speed-of-sound on environmental variables such as temperature and humidity is well understood. Sound involves the compression and expansion of air. There is a restoring force that tends to return the pressure p (and density xcfx81) back toward the static value. As the air expands and contracts, the compression and expansion causes it to heat and cool, respectively. The expansion and contraction occurs fast enough so that on the distance scale of a half wavelength, heat flow is negligible between adjacent volumes of high and low pressure. Consequently,                                           c            2                    =                      (                                          ∂                p                                            ∂                ρ                                      )                          ,                            (        1        )            
where the partial derivative refers to a process with zero heat flow through the sample""s boundary. For a gas sample taken through such a process, the result is,
c={square root over (xcex3RT/xcexc)}xe2x80x83xe2x80x83(2)
where R is the ideal gas constant, T is the absolute temperature (in K), xcexc is the average molecular weight of the gas molecules, and
xcex3=cp/cvxe2x80x83xe2x80x83(3)
where cp is the heat capacity of the gas at constant pressure and cv is the heat capacity of the gas at constant volume.
Inserting known values for dry air, R=8.31447 J/(mol K), and xcexc=2.8964xc3x9710xe2x88x922 Kg/mol. At 300 K, xcex3=1.4000 and at 400 K, xcex3=1.3951. Approximating xcex3(T) as a linear function that passes through these two points, in dry air at 20xc2x0 C. (293.15 K), xcex3=1.4003 and, by Eq. (2) c=343.3 m/s. In comparison, the measured c in dry air at 20xc2x0 C. is 343.6 m/s.
Therefore, from Eq. (2), c is about proportional to {square root over (T)} with T in Kelvin (T[K]=T[xc2x0C.]+273.15). This temperature dependence is the basis of the air temperature measurement. A change of air temperature by 1xc2x0 C. from 20 to 21xc2x0 C., increases c by a factor of 1.00169. Thus, to determine T to an accuracy of 1xc2x0 C., c must be measured with relative accuracy of about 1.7xc3x9710xe2x88x923.
The expression for c in Eq. (2) is independent of pressure p at constant T. This simplifies the determination of T from measured c; the relationship does not involve air pressure, so p does not need to be measured.
The present invention is directed to a control system that uses an acoustic transducer to determine the average air temperature along a path within a vehicle""s interior. The measured temperature is used as an input to the vehicle""s automatic climate control (ACC) system.
The present invention may be used to acoustically control an automatic climate control system for a vehicle""s interior by determining an average air temperature adjacent to a vehicle""s occupant using at least one ultrasonic transducer. The transducer transmits and receives an ultrasonic pulse along a pre-selected path within a vehicle""s interior and concurrently measures the propagation time along the path. A signal processing circuit is then used to compute a signal which is related to the average air temperature along the acoustic path. For example, this signal may be an analog voltage which is proportional to the average air temperature, or a digitally encoded format may be used. In either case, the ACC controller is programmed to recognize the signal representing the determined average air temperature. The determined average air temperature is evaluated against a pre-selected setpoint temperature using a controller to determine whether to activate the blower fan, the air conditioner or the heater for a sufficient period of time to adjust the vehicle""s interior air temperature to a desired temperature.
The change in the speed of sound caused by air velocityxe2x80x94the Doppler effectxe2x80x94also needs to be considered. If sound propagates a distance d in one direction from a source to a receiver, and if the component of air velocity along the vector from the source to the receiver is u, then the propagation time is                               Δ          ⁢                      xe2x80x83                    ⁢          t                =                              d                          c              +              u                                .                                    (        4        )            
Thus, based on a measurement directly along a straight line from a source to a receiver, the effect of the component of air velocity along the path is simply to add to c. To cause an apparent 1xc2x0 C. change in T, the flow velocity is 0.6 m/s. For comparison, the air velocity from the AC outlets at high fan level is typically 5 m/s. The air velocity near an occupant is typically less than 1 m/s, and this would change measured temperature on the order of 1xc2x0 C.
The Doppler effect cancels out to first order if the sound propagates from a source to a reflector and then returns back along the same path to the source where it is detected. With a constant u along the entire path                               Δ          ⁢                      xe2x80x83                    ⁢          t                =                                            d                              c                +                u                                      +                          d                              c                -                u                                              =                                    2              ⁢              d                                      c              -                              (                                                      u                    2                                    /                  c                                )                                                                        (        5        )            
Thus, if the same transducer is used to send and receive ultrasonic pulses, the effect of u is to change the apparent c by u2/c. If u is 5 m/s and c is 345 m/s then the apparent change in c is 0.072 m/s. This is equivalent to a change in T of only 0.12xc2x0 C.
This shows that the effects of air velocity on the measurement of temperature are extremely small for cases of practical interest, especially if the velocity of sound is averaged over both directions along a pre-selected path. In some cases, this may not be necessary. However, if the interval times for ultrasonic pulses traveling in opposite directions along the same pre-selected path are measured, then the average air velocity along this path, u, can be determined.